This October 2007 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.
Aircraft: Cessna 172. Location: Prescott, Ariz. Injuries: 1 Minor. Aircraft damage: Substantial.
What reportedly happened: According to the pilot, the plane was in cruise flight when the engine lost power and made a clanging noise. The engine continued to operate at a maximum of 900-1,100 rpm. The pilot was not able to maintain altitude. He turned toward the airport and reported his situation to the control tower. He determined that he would not be able to glide to the runway, so he turned toward a road for an emergency landing. Due to powerlines and a blind curve on the paved road, he opted to land adjacent to the road. The airplane hit a ditch and flipped over.
The engine had been overhauled two flight hours before the accident. The field overhaul had been performed on the engine due to a bent pushrod, stuck valve, and broken lifter for the number 3 cylinder. During the field overhaul, all four of the rocker arms had been magnetic particle inspected. The post-accident examination of the engine revealed that the number 3 cylinder exhaust rocker arm had separated into two pieces. A metallurgical examination determined that the number 3 exhaust rocker arm had fractured around the central pivot hole due to fatigue. The fatigue originated at the outer surface of the oil hole. The corners of the hole were sharp and had not been de-burred, which increased the likelihood of fatigue cracking. The NTSB metallurgist also noted staining on the cracks from the solution used during the magnetic particle inspection. Based on the staining, the cracks would have been detectable in size (about 0.2 inches) at the time of the inspection.
Probable cause: The failure of maintenance personnel to detect fatigue cracks in the number 3 exhaust rocker arm, which led to a partial loss of engine power.
For more information: NTSB.gov